Add sqlalchemy subclass for Application

Add an implementation package for the base object API that reuses
sqlalchemy ORM for model definition, but which provides its own
object level methods for persistence (create, save, and delete).
Most object-impls can then share common code, with a few callouts
explicitly intended to support live schema migration.

The default backend object implementation for Solum is
solum/objects/sqalchemy.

Change-Id: Icbf53ff07097e4c74c4fef155cc0bf5bc54efc18
Ensure object model is loaded at correct time

Move common service startup code to solum/common/service for use
in other commands.  Object model is then loaded after configuration
is parsed so that subclasses can choose behavior based on the config,
for instance, whether the new or old schema is in place.

Change-Id: Icb692e40eaa65b545eae916bdd6c84396ea0e625